The striking white calf was spotted by Otago University researchers playing alongside other southern right whales.

"They're really quite ethereal when you see them - not just from the surface, but especially from the drones," marine mammal scientist David Johnston said.

"It makes it really easy to track them, but also identify them as well."

The research team spent a month in the sub-Antarctic islands, getting a new perspective on the recovering population of southern right whales using drones.

"It's really the first time we can collect a whole heap of really high-quality images of whales from above," Mr Johnston said.

"Traditionally we've had to rely on helicopters or small aircraft, which are really expensive things to run."

The adapted drone system enabled the team to gather accurate measurements of the width and height of the whales.

"Obviously if the whales are big and fat and happy and healthy, it tells us that the population is in good shape, and that they're feeding up large in the Southern Ocean during the summertime," University of Otago marine science lecturer Dr Will Rayment said.

It was a southern right whale which entertained residents in the capital in July.

The large animal frolicked in Wellington Harbour for days, with crowds of whale-watchers coming down for a look.

Researchers say it's an encouraging sign that more of the whales are visiting mainland New Zealand as their numbers increase.

"It's really important that as the population recovers and starts to recolonise New Zealand's coastal waters, that we understand what's happening to that population in the subantarctic," Dr Rayment said.

They'll also be comparing how our southern right whales are faring in relation to similar "at-risk" populations around the world.